In larger power distribution systems it is frequently necessary to switch ungrounded three-phase delta-connected systems. This is particularly true in systems feeding critical loads, where an alternate three-phase source may be switched to in the event of failure of a primary system.
Prior to switching to such an alternate source it is desirable to ascertain that the alternate source is at the correct voltage level. Because of the high operating voltages involved and the necessity of maintaining electrical isolation between the phases of the alternate source and ground, this has in the past been difficult and expensive to accomplish. Typically, individual potential transformers have been required in association with each phase, together with extensive monitoring equipment for analyzing the output of the transformers.
In contrast, the present invention provides a self-contained voltage monitor which may be installed over existing conventional test points associated with an ungrounded delta-connected three-phase system and does not require the use of potential transformers and associated monitoring circuitry. In the event of a low voltage condition, and, if desired, in the event of a high voltage condition, the monitor provides a control signal which can be utilized to sound an alarm, or to inhibit switching to the alternate source, either locally or at a remote location.
Thus, the present invention provides a voltage monitor particularly well suited for monitoring the phase-to-phase voltage of an ungrounded delta-connected power system of the type commonly used for supplying electrical power to large industrial loads.
Accordingly, it is a general object of the present invention to provide a new and improved voltage monitor for an ungrounded delta-connected three-phase electrical power system.
It is a more specific object of the present invention to provide a voltage monitor for an ungrounded three-phase power system wherein electrical isolation is maintained between the phases of the system and electrical ground.
It is a further object of the present invention to provide a three-phase voltage monitor which is reliable in operation and economical to construct and which may be conveniently installed on the conductors of an existing three-phase system.